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14 Journal Articles
11 Other Resources
Journal Articles: First 3 results.
Pedagogies:
Percy Julian, Robert Robinson, and the Identity of Eserethole  Addison Ault
The Nova production Percy JulianForgotten Genius, which included the very public disagreement over the identity of "eserethole," the key intermediate for the synthesis of the alkaloid physostigmine, left three important chemical questions unanswered.
Ault, Addison. J. Chem. Educ. 2008, 85, 1524.
Constitutional Isomers |
Enantiomers |
Natural Products |
Stereochemistry |
Synthesis
Using "Basic Principles" To Understand Complex Science: Nicotine Smoke Chemistry and Literature Analogies  Jeffrey I. Seeman
The HendersonHasselbalch equation calculates the equilibrium distribution of 50:50 for nicotine in its nonprotonated (free base form), relative to its monoprotonated form, at pH of 8 in dilute aqueous solution. This ratio has then been used in the literature to predict the effect of ammonia compounds in tobacco and in smoke on nicotine pyrolysis and smoke chemistry. Experiments demonstrate that neither the thermal chemistry of tobacco alkaloids nor the transfer of nicotine from tobacco to smoke can be explained by the position of the nonprotonated versus monoprotonated form equilibrium in aqueous extracts of tobacco. The high thermal stability of nicotine in air allows nicotine salts to be converted to nonprotonated nicotine and volatilize during heating prior to any substantial decomposition of the nicotine moiety. In contrast, cocaine hydrochloride is thermally unstable and will rapidly decompose upon heating; cocaine hydrochloride must first be converted to its nonprotonated form prior to heating and volatilization.
Seeman, Jeffrey I. J. Chem. Educ. 2005, 82, 1577.
Acids / Bases |
Applications of Chemistry |
Calorimetry / Thermochemistry |
Heterocycles |
pH |
Natural Products |
Gases
Isolation of Shikimic Acid from Star Aniseed  Richard Payne and Michael Edmonds
The isolation of shikimic acid from star aniseed is described. This experiment uses Soxhlet extraction followed by ion exchange chromatography to isolate crude shikimic acid from the star aniseed. Subsequent charcoal decolorization and recrystallization from toluene and methanol afford the pure shikimic acid in 27% w/w yield.
Payne, Richard; Edmonds, Michael. J. Chem. Educ. 2005, 82, 599.
Bioorganic Chemistry |
Natural Products |
NMR Spectroscopy |
Plant Chemistry
View all 14 articles
Other Resources: First 3 results
Molecular Models of Natural Products  William F. Coleman
This month's issue of the Journal includes several papers discussing interesting molecules that fall into the broad category of natural products, and four of these papers serve as the source for our featured molecules this time around. Addison Ault weaves an interesting tale of the search for the true structure of eserethole and of the competition between two research groups that of Percy Julian, consisting of two people, and that of the British chemist Robert Robinson, a large group at Oxford (1). David Vosburg describes a case study approach to teaching organic synthesis and includes a number of molecules that have been the basis of student research papers (2). Jean-Michel Lavoie, Esteban Chornet, and André Pelletier have developed an experiment utilizing GCMS to separate terpenes from citrus (3), and Patty Feist, in a paper that may send readers running for their Kafka, has students synthesize a cockroach pheromone that may have wide applicability in cockroach control without the problems created by many insecticides (4).The molecules that have been added to our collection contain a wide variety of functional groups, and would serve as a good source for an exercise in having students recognize these functional groups in a number of different settings. Questions such as How many cyclic ether groups are present?, How many bridgehead carbons?, or How many chiral centers would be useful exercises in organic and introductory non-majors courses. Students could find other pheromone structures and see how they compare with that of blattellaquinone, or explore the various ways in which the steroid backbone shows up in the collection.This collection of molecules also provides a good starting point for students to use the capabilities of Jmol to further explore structural features. The focus here is on measuring bond distance and angles. Double clicking on any atom will change the cursor to a cross-hair (this may take a little practice). One end of a dashed line is now locked to that atom. Dragging the free end of the line to other atoms will show the distance between the two centers in nanometers. Double clicking on a second atom will lock a line segment between those two atoms and display the distance in black. There is now another free end of the segmented line, and dragging that to any other atom will show the angle defined by the three-atom combination. Double clicking on the third atom fixes the second line segment and gives a third segment that can be dragged and double clicked to display dihedral angles. Students could, for example, explore various ring structures in this collection to determine which rings are distorted and which are not.The files that are currently used for the collection are MDL mol files, and do not contain orbital, electrostatic potential, or vibrational data. Beginning next month we will change the file format, and that information will be available to users, either through the Jmol menu (right click on any structure) or through menu choices.Not all of the molecules from the Ault paper (1) have been included, leaving room for students to model and perform calculations on many of the non-eserethole species, and to consider how modern tools of analysis might have simplified the identification of eserethole. They might also wish to determine which pair of eserethole enantiomers are the more stable. (The eserethole structures included here have all been optimized at the 6311++G (d,p) level.)
Natural Products
Molecular Models of Rosmarinic Acid and DPPH  William F. Coleman
The paper by Canelas and da Costa (1) introduces students to the antioxidant rosmarinic acid, and its interaction with the free radical DPPH. Those two molecules are the featured species this month. The original paper shows the 2-dimensional structure of the cis isomer of rosmarinic acid, although the trans isomer exhibits very similar antioxidant properties. Calculations at the DFT/B3LYP 631-G(d) level show that the trans isomer is more stable than the cis isomer in the gas phase, a situation that is expected to carry over into solution. Many antioxidants are phenols, and rosmarinic acid has four such groups available for radical formation. A DFT study by Cao et al. (2) examines the relative stabilities of the radicals formed from loss of each of the phenolic hydrogens. That paper focuses on the trans isomer, and a useful student project would be to repeat the calculations with the cis isomer. An HPLC separation of the isomers of rosmarinic acid has been published (3), and might well lead to an extension of the experiment described in ref 1 in which relative antioxidant efficiencies of the two isomers could be evaluated. DPPH has been used extensively as a standard for determining antioxidant activity. An examination of the molecular orbital occupied by the lone electron shows significant delocalization, providing a partial explanation for the stability of the neutral radical. Our gas phase structure for DPPH, also at the DFT/B3LYP 631-G(d) level, is quite consistent with several crystal structures on DPPH and DPPH in the presence of another species (4).
Natural Products
Molecular Models of Resveratrol  William F. Coleman
The featured molecules this month are from the paper "Resveratrol Photoisomerization: An Integrative Guided-Inquiry Experiment" by Bernard, Gernigon, and Britz-McKibbin exploring trans to cis photoisomerization in resveratrol. Examination of Figure 1 in that paper, where the hydrogen atoms have been omitted, might lead one to conclude that the structures are relatively straightforward. These isomers provide students an excellent opportunity to test their ability to take a two-dimensional representation and envision the three-dimensional structure of the molecule and to consider the competing factors that might lead to the three-dimensional structures being non-planar. The two-dimensional models focus attention on the possibility of extended pi-electron delocalization. Addition of the hydrogen atoms clearly suggests that delocalization will compete with non-bonded H-H repulsions in the cis isomer. Further examination of the trans isomer shows that such non-bonded interactions are, in what one might call a first-order approximation, like those in biphenyl interactions that lead biphenyl to be non-planar in both the gas phase and in a variety of solvents. The backbone of the trans isomer of resveratrol, trans-stilbene, has been the subject of a number of theoretical and experimental investigations (1, 2). In general, Hartree-Fock calculations predict a non-planar geometry for this molecule while Density Functional Calculations, using the same basis sets, predict an essentially planar structure. Spectroscopic evidence supports a temperature-dependent structure for trans-stilbene with the molecule being planar at low temperature and non-planar at high temperatures. Our calculations on trans-resveratrol produce similar results. Hartree-Fock calculations using the 6-31G** (6- 31G(d,p)) basis set predict a dihedral angle of approximately 24 degrees between each ring and the central carbon-carbon double bond. This result is consistent with the reported value of 23 degrees using the 6-31G* basis set. We also find that DFT calculations using the B3LYP functional and the 6- 31G** basis set, lead to a planar configuration. We include several versions of trans-stilbene and trans-resveratrol in the molecule collection so that students can explore these structural questions in more detail. For each molecule, structures obtained from PM3, HF(6-31G**), and DFT(B3LYP/6-31G**) calculations are included, as well as planar and non-planar structures of biphenyl. Measurement of the various bond and torsion angles using Jmol will help students develop a sense of the distance dependence of the non-bonded interactions and their importance in determining the actual structure. They might also wish to consider what additional degree(s) of freedom resveratrol and stilbene have that biphenyl does not, allowing the trans-form of the former molecules to remain planar under certain conditions, while minimizing the effect of the non-bonded repulsions.
Plant Chemistry |
Natural Products
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